The present invention relates to manufacturing of micro-electromechanical devices (MEMS), and more specifically to methods for forming a component of MEMS inductors, transformers, inductive actuators and/or inductive sensors.
High performance soft magnetic materials can store magnetic energy, which can be used to build inductors or transformers for radio frequency circuits or power electronics. These soft magnetic materials can also concentrate, shape and guide magnetic flux, which can be used to form inductive MEMS structures. These inductive MEMS structures can therefore interact with a magnetic field, the mechanism of which can be used to build MEMS magnetic sensors and electromagnetic micro-actuators and/or generators.
MEMS inductors are usually composed of a conductive coil (e.g., copper lines) which carries currents and a magnetic core that stores magnetic energy. Material used in forming the magnetic core materials used in MEMS, usually Ni—Fe or Co—Fe based alloy, is generally deposited via electroplating or sputtering techniques. Electroplating requires external electrodes and a seed layer to carry current to produce the necessary electrochemical reactions. For wafer-level integration, these requirements are met by making electrical contacts at the edge of a silicon wafer having the MEMS inductor and a thick seed layer to ensure uniform current distribution across the entire wafer. This can be a challenge for large scale wafers and for three-dimensional MEMS structures. Additionally, most electroplating materials, such as Ni—Fe and Co—Fe, have a low resistivity (e.g., <45 micro-Ohm-centimeters). However, the low resistivity of these materials limits their application at high frequencies (e.g., >10 Megahertz). Sputtering, on the other hand, usually produces low deposition rates and generally does not provide conformal coverage. Additionally, magnetic films derived from sputtering are difficult to pattern subtractively due to the challenges of mask alignment and long etching times. Thick film sputtering processes require high vacuum and frequent system maintenance. thereby making sputtering processes expensive and impractical with respect to integration and manufacture of the magnetic cores, etc. The deposition process can result in high stress on a wafer, and, in particular, on large scale wafers (e.g., >200 mm).